1. Field of the Invention
The present invention relates in general to the field of generation of precise electrical signals, in particular, to a technique for providing accurate delay of signals using a controllable delay line, and is applicable to the areas of high speed communication and memory testing equipment.
2. Background Art
As the transmission rate of the data communication signals in modern electronic systems continues to increase, new circuitry and methods are needed to accurately clock command data and create precise controllable delays because it is often desired to delay signals at time intervals much shorter than one period of the master oscillator. For example, the master oscillator might have a frequency of 100 MHz. Such an oscillator has a period of 10 nsec. However, the communication link might have a data rate of 2 Gbits per second. Thus, it may be required to place timing edges with an accuracy of 50 psec and even higher.
This is typically implemented by using a delay line to form a timing vernier as is shown in the prior art, e.g. in U.S. Pat. No. 5,963,074. According to this procedure, a signal passes through the delay line which has taps spaced closely apart in time. By selecting the appropriate taps several signals with intermediate timing can be generated. The system can include a programmable encoder, so that each selectable delay can be separately calibrated.
Known are programmable phase shifters for phase shifting a periodic input signal by an adjustable phase angle to produce an output signal, such as described in U.S. Pat. No. 5,994,938. The phase angle is controlled by, for example, 10-bit input control data. The phase shifter includes a tapped delay line formed by a set of differential or single-ended buffers connected in series for successively delaying the input signal to produce a set of “tap” signals at the buffer outputs.
A programmable phase shifter disclosed in U.S. Pat. No. 6,104,223 includes a tapped delay line, a multiplexer, a programmable delay circuit and, additionally, a programmable data converter which converts input data for controlling multiplexer selection and the amount of delay provided by the programmable delay circuit.
However, as integrated circuit technology has progressed and demanded higher speed, more accurate and more complicated communication equipment, a problem arises to be applied to the timing vernier circuitry is that logic propagation delays are dependent upon the power supply voltage and the temperature of the silicon substrate.
Though the supply voltage sensitivity can be managed to some extent by very tightly regulating the supply voltage, this adds cost to the test system, and also the temperature of the substrate is much more difficult to control.
Additionally, the problem of variable environmental parameters can be solved as suggested in U.S. Pat. No. 6,092,030, when the effect of the significant environmental variables is measured and an adjustment made to the delay circuitry to compensate for the changes in the environmental variable is stored in a memory. Circuitry is designed into the integrated circuit that will measure the appropriate environmental variable or variables. The results of the measurement are used to adjust the delay so that the timing edges are in the correct location with respect to the master oscillator.
When the various delays provided by a programmable delay circuit are adjustable, the phase shifter can be calibrated by measuring the phase shift and adjusting the delay provided by programmable delay circuit. However, the above technical solution requires continuous monitoring and updating compensation values and also frequent re-calibration of adjustment coefficients to provide accurate timing which is a costly and time consuming operation affecting timing accuracy.
It is therefore an object of the present invention to provide a system and method for supplying accurate and constant time delays which do not depend on such environment parameters as temperature variations and supply voltage fluctuations and do not require continuous update, but allows for simple and cost-effective maintenance.